Among prior art well-known silicone self-adhesive compositions is a composition comprising a condensate between a polysiloxane having (CH.sub.3).sub.3 SiO.sub.1/2 and SiO.sub.2 units and a dimethyl-silicone raw rubber. This self-adhesive composition is crosslinked with peroxides for the purpose of enhancing adhesive force and cohesive force whereas the curing temperature is as high as 180.degree. to 200.degree. C. There is a need for low-temperature curable silicone self-adhesive compositions from the standpoints of energy cost, productivity and application to substrates having a low heat distortion temperature.
Japanese Patent Publication No. 37907/1979 proposes a low-temperature curable silicone self-adhesive composition utilizing addition reaction between an alkenyl group-containing organopolysiloxane having a viscosity of 200 to 1,000 poise and a SiH-containing organopolysiloxane. This self-adhesive composition, however, is difficult to form a layer having good physical properties and consistent adhesive force on substrates. To obviate these drawbacks, Japanese Patent Application Kokai No. 22886/1088 proposes a silicone self-adhesive composition utilizing addition reaction between an alkenyl group-containing organopolysiloxane having a viscosity of at least 500,000 centipoise (raw rubber) and an SiH-containing organopolysiloxane. The composition is alleged to show satisfactory, consistent adhesive force. However, when exposed to elevated temperatures beyond 200.degree. C. for an extended time, this self-adhesive composition gives rise to the problems of foaming from the raw rubber triggered by cracking, a limited adhesive area, and a noticeable loss of adhesive force, with the additional drawback of a product value lowering due to deteriorated appearance. The composition is not suitable for heat resistant adhesive tapes, labels and other applications where it should withstand elevated temperatures for a long time.
Therefore, an object of the present invention is to provide a silicone self-adhesive composition which not only meets the general demand for brief curing at low temperatures, but also forms a heat resistant silicone adhesive layer which has excellent adhesive properties and undergoes no foaming even after long-term exposure to elevated temperatures.